Phosphor slurry coating applied to an LED can be a representative example of coating material because it is difficult to treat, requires a high degree of uniformity, and enjoys advantages of uniformity. In conventional methods of manufacturing white light LEDs, a slurry containing at least one kind of phosphor such as YAG, TAG, or silica-based material and a binder mixed together is dispensed onto an ultraviolet or blue light emitting diode using a dispenser to coat it, a like slurry further containing a solvent added to reduce the viscosity is directly sprayed to the LED using an ultrasonic atomizer, which is a kind of fine particle generating device, or a spray device to coat it, a phosphor plate or film is prepared to cover the LED, or a phosphor sheet called a remote phosphor is prepared and attached at a location remote from the LED.
Patent Literature 1 discloses a method for manufacturing an LED by circulating a slurry using a circulator and applying the slurry containing a phosphor to a heated LED chip by spraying while whirling the slurry in a spiral manner with compressed air, thereby applying the slurry to a side surface of the LED, which is considered to be difficult to coat by common spraying methods.
Patent Literature 2 discloses a process of coating an LED chip with a binder such as silicone and curing it, applying a slurry containing a phosphor, a binder, and a solvent, which may be mixed with a diffuser if necessary, onto it to form a lamination while making a pass/fail judgment by measuring the color temperature.
Patent Literature 3 discloses a process of transferring a slurry composed of a phosphor, a binder, and a solvent having a viscosity between 0.1 and 200 centipoises between two syringes, applying teaching of Japanese Patent Application Laid-Open No. 2004-300000, and applying the slurry to a chip multiple times while whirling the spray stream employing an air pulse spray taught by Japanese Patent Application Laid-Open No. 59-281013.
Patent Literature 4 discloses an apparatus in which a viscous material having a viscosity effectively higher than 100 centipoises (100 mPa·s) supplied from a reservoir through a metering device such as an auger valve is dispensed through a dispensing element (needle), and a measuring apparatus receives and measures a metered quantity of viscous material to adjust the moving speed of a robot and the discharge speed of the viscous material through the needle.
A method using a dispenser as disclosed in Non-Patent Literature 1 is widely employed to fill a cup in which a chip is mounted with the slurry for mass production of artillery-shell-shaped LEDs of not high power and LEDs for back light for TV sets or PCs.
It is true that the method according to Patent Literature 1 is ideal in the aspect that sedimentation of phosphor particles is harder to occur than in the case with common apparatuses because of the circulation of the slurry containing the phosphor particles, but the method uses a large circuit and needs a material several to twenty times as expensive as a material needed for application. Moreover, because the circuit is complex, a slight amount of sedimentation and clogging occur at many locations in the circuit, leading to instability in the coating amount.
In the method disclosed in Patent Literature 2, an LED chip is coated with a binder, then it is cured, and thereafter a slurry containing phosphor is applied thereon by air spraying to form a lamination. Moreover, the color temperature is measured during the process, and the process is allowed to proceed to the next step only when it satisfies a criterion. However, measuring the color temperature in the production line on a layer-by-layer basis requires a complex process and expensive equipment. Moreover, it is necessary to process LED chips one by one, taking too long time.
The method disclosed in Patent Literature 3 is suitably employed in the R & D or in cases where middle scale production is desired, because of the efficient circulation of a small quantity of material. However, making the set air pressure low to decrease the circulation speed causes sedimentation, and making the air pressure high to increase the circulation speed leads to higher shear than necessary acting on the slurry, whereby the viscosity is decreased with the lapse of time, and the quantity of the applied material tends to increase. Therefore, the weight is checked frequently by manual operation during the coating application to the work.
On the other hand, in case where a slurry without solvent containing a binder such as silicone and phosphor particles is applied through a dispenser using a simple apparatus like one disclosed in Non-Patent Literature 1, sedimentation tends not to occur because of relatively high viscosity. Nevertheless, sedimentation eventually occurs with the lapse of time. Therefore, a positive displacement pump such as a small-size auger pump or a plunger pump, which enables easy control of the coating weight, is commonly employed. However, the specific gravity changes with sedimentation, leading to unevenness of quality. In order to improve the quality no matter how small a degree is, coating operation is performed while checking the weight using a measuring device in the coating booth applying the teaching of Patent Literature 4, in some cases. However, in the case where a slurry containing a volatile component such as a solvent is applied by spraying or the like process, the coating booth needs ventilation. Specifically, it is mandatory that the face velocity near the spray booth door be 0.4 meter per second or higher. Therefore, the fast air flow adversely affects the measuring device and the spray flow gives an impact on the measurement plate or the like, which leads to another problem.